Giving lettes an individual node
First go to Material Tab, then click the + button next to your material, now press the 'Brows Material to be Linked' button and finally press the 2 button next to the material name. Now you have to go to edit mode and click the Assign button at the bottom of the materials list while the letter you want to give the particular material is selected.
By adjusting the new material you will now change the previous
Just see the Gif.